1. Field of the Invention
This invention is in the field of pressurized furnace steam boilers and also the field of coal gasifier processes and apparatus utilizing cyclic compression and expansion to force reactant gases into the coal pores and to expand reacted gases out of the coal pores.
2. Description of the Prior Art
The common form of pressurized furnace steam boiler is the Velox boiler wherein an air compressor delivers compressed combustion air into a sealed and pressurized furnace where combustion with added fuel occurs. The combustion gases are cooled when passing over the boiler heat transfer surfaces and steam is generated at pressure inside the boiler. After being thusly cooled, these combustion gases are then expanded through a gas turbine engine whose work output is used to drive the combustion air compressor. By use of adequately high combustion pressures, the net work of the gas turbine engine can exceed the work input to the air compressor and a net useful work output results. As compared to the more usual atmospheric pressure furnace steam boiler, the Velox boiler has the advantages of a smaller size for a given capacity and the possibility of generating a net useful work output, whereas the atmospheric furnace boiler requires some work input to drive the forced and induced draft fans. Descriptions of prior art Velox boiler schemes are presented in references A and B.
The Velox boiler is a special form of combined cycle power plant. The more common form of combined cycle power plant uses a gas turbine cycle with a combustion air compressor, an uncooled combustion chamber, and a gas turbine expander engine, and the expanded gases leaving the gas turbine expander engine are then passed through the atmospheric pressure furnace of a steam boiler to generate steam at pressure for a steam power cycle. Because the gas turbine cycle combustion chamber is uncooled, considerable excess air must be used as a coolant to keep the combustion gas temperatures below those producing damage or deterioration of the gas turbine expander engine. The necessary compression and expansion of this excess air for cooling reduces the efficiency of this common form of combined cycle steam power plant. With a Velox boiler form of combined cycle power plant, the gas turbine cycle combustion chamber is cooled by the steam boiler and excess cooling air is thus not needed and the resulting excess air efficiency losses are avoided in this way.
Combined cycle power plants of either the common type or the Velox boiler type are today essentially limited to using liquid fuels or gaseous fuels and these fuels are today much more costly than coal. When pulverized coal is used in combined cycle power plants, serious blade erosion occurs in the gas turbine expander engine due to the solid ash particles produced.
Efforts to burn coal in lump form in combined cycle power plant gas turbine cycle combustion chambers, in order to avoid the blade erosion problem, have encountered the following problems instead:
a. it is difficult to feed lump coal into combustion chambers, which are always pressurized, by use of prior art lock hopper valves;
b. lump coal bed burning tends to produce channeling and thus to maldistribute the air flow over the fuel lumps, a slight excess of air in one area burning the coal there more rapidly and thus causing yet more air to flow through this consequently reduced flow resistance channel, resulting in still more rapid coal burnup there;
c. fuel spreaders or a moving fuel bed grate are usually required to avoid excessive combustion air channelling and these are difficult to operate and maintain inside a pressurized combustion chamber.
Some of the problems of lock hopper valves when operated at pressure are described in reference C.
As a result of these dificulties with both pulverized coal firing and lump coal firing, few combined cycle power plants now use coal as fuel despite the greatly lower cost of coal. Nor are many new combined cycle power plants likely to be built despite the appreciable improvement in plant efficiency of combined cycle plants since the only useable fuels, gas and oil, have become too costly.
Lump coal is commonly burned in moving beds in steam boilers whose furnaces operate at essentially atmospheric pressure by use of moving grates. Even at atmospheric pressure, however, moving grates are a maintenance problem due to the high temperatures at which they operate and the necessity for motion of the grate. Beds of lump coal are usually capable of removing a higher proportion of the sulfur dioxide from fuel sulfur burnup than are pulverized coal burners due to the closer contact of the sulfur dioxide gas with either basic ash ingredients from the coal or with added basic materials such as dolomite.
It would thus be of great benefit to have available for use in combined cycle power plants a combustion scheme for use in pressurized combustion chambers which could burn lump coal in a fixed bed without the need for fuel spreaders or moving grates and whose lock hopper valves could be operated at low pressures.
The term water is used herein and in the claims to mean either liquid water or steam, which is defined as water vapor, or a mixture of liquid water and steam.
The term boiler means is used herein and in the claims to mean an enclosed pressure vessel with liquid water inlet and steam outlet and with heating surfaces for boiling the enclosed liquid water flowing into the liquid water inlet and for heating the resulting steam at pressure above atmospheric and comprising the usual steam boiler auxiliaries such as pressure relief valves, water level indicators if useable, exterior insulation, etc. Those heating surfaces of a boiler means which directly view a combustion chamber or solid fuel bed are herein and in the claims referred to as radiant heaters since significant heat transfer can occur to these surfaces by radiation from the burning fuel as well as by convection from combustion gases. Those heating surfaces of a boiler means which do not directly view a combustion chamber or solid fuel bed, or whose view thereof shows only small area, are herein and in the claims referred to as convection heaters since most of the heat transfer to these surfaces occurs by convection from hot combustion gases. A boiler means may comprise but a single heating surface, either a radiant heater or a convection heater, but usually more than one heater is used and both radiant heaters and convection heaters are frequently used in combination in a single boiler means. A boiler means can be of the once-through type with liquid water entering the boiler liquid water inlet and the water flows unidirectionally through the boiler heating surfaces toward the boiler steam outlet where the water emerges as steam. Alternatively, a boiler means can be of the separator and recirculator type wherein liquid water and steam pass into a steam and liquid water separator, such as a steam drum, after passing through a principal portion of the boiler heating surfaces. The liquid water separated by the separator is then recirculated back through the same principal portion of the boiler heating surfaces, either by a forced recirculator pump or by a natural convection recirculator. The steam from the separator continues on to superheater surfaces for further heating or to the boiler steam outlet.
Since boilers usually operate at pressures well above atmospheric, a feedwater pump and feedwater pump drive means are used to pump liquid water into the boiler liquid water inlet at a feedwater flow rate equal to the rate at which steam is being formed within the boiler and is leaving via the boiler steam outlet. Various types of feedwater pump flow rate controls are used to insure an adequate flow of liquid water into the boiler to prevent overheating of any of the heating surfaces of the boiler means.
The term superheater means is used herein and in the claims to mean an enclosed pressure vessel with steam inlet and superheated steam outlet and with heating surfaces for superheating the enclosed steam flowing into the steam inlet at pressures above atmospheric. Superheaters can be of the radiant heater type or of the convection heater type or of both types together in combination.
The term reheater means is used herein and in the claims to mean an enclosed pressure vessel with steam inlet and reheated steam outlet and with heating surfaces for reheating the enclosed steam flowing into the steam inlet at pressures above atmospheric. Reheaters can be of the radiant heater type or of the convection heater type or of both types together in combination.
Superheaters and reheaters are very commonly used in fossil fuel fired steam electric power plants in order to keep the steam free of liquid water as it expands through the steam turbine so that blade erosion by liquid drops can be avoided.
The steam side of a boiler means, a superheater means or a reheater means is that side of the pressure vessel in contact with water. The gas side of a boiler means, a superheater means or a reheater means is that side of the pressure vessel in contact with combustion gases or viewing the combustion chamber or fuel bed or both.
The term oxygen gas is used herein and in the claims to mean oxygen molecules not combined with any other chemical elements. Air, for example, is a gas containing appreciable quantities of oxygen gas. Carbon dioxide, on the other hand, is a gas devoid of oxygen gas even through oxygen atoms exist therein combined with the carbon atoms.
The term char fuel is used herein and in the claims to include any carbon containing fuel which is either a solid or can be transformed at least partially into a carbonaceous solid when volatile portions thereof are removed. Included as char fuels within this definition are coal, coke, wood, wood charcoal, oil shale, petroleum coke, garbage, wood bark, wood wastes, agricultural wastes and other carbonaceous materials as well as mixtures of these fuels.